1. Field of the Invention
The present invention relates to a scanning exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
A projection exposure apparatus has been conventionally employed in manufacturing micropatterned semiconductor devices such as a semiconductor memory and a logic circuit using photolithography. The projection exposure apparatus projects and transfers a circuit pattern drawn on a reticle (mask) onto a substrate such as a wafer by a projection optical system. These days, to keep up with the demands for advances in micropatterning of semiconductor devices, the projection exposure apparatus has achieved a high resolution to the degree that it forms by exposure a pattern with a feature size that is half the exposure wavelength or less, leading to ceaseless demands for further improvements, that is, increases in its resolution.
An increase in the resolution of the exposure apparatus is generally achieved by shortening the wavelength of the exposure light, and increasing the numerical aperture (NA) of the projection optical system. However, the shortening of the wavelength of the exposure light and the increase of the NA of the projection optical system are insufficient to achieve an increase in the resolution of the exposure apparatus, and the performance of the exposure apparatus itself must improve. For example, the recent projection optical system suffers much less aberration than ever. In addition, the conventional illumination optical system has been allowed to use non-polarized illumination alone, but the recent illumination optical system realizes polarized illumination. Herein, polarized illumination means an illumination scheme which controls the polarization state of the illumination light in accordance with the reticle pattern.
In order to continue to upgrade the exposure apparatus, it includes a measurement device which measures various types of optical characteristics, and a correction unit which corrects the optical characteristics based on the measurement results obtained by the measurement device. For example, an exposure apparatus which can measure and correct the aberration of a projection optical system as mounted in the exposure apparatus has been proposed. Examples of measurable optical characteristics other than the aberration of the projection optical system are the NA of the projection optical system, the polarization state of the illumination light, the curvature of field, the exposure amount, the uniformity of the exposure amount, the illumination light distribution, and the Jones matrix of the projection optical system.
Exposure apparatuses are roughly classified into an exposure apparatus of the step & repeat scheme (a stepper), and an exposure apparatus of the step & scan scheme (a scanner). A scanner is more suitable for coping with an increase in the NA than a stepper, and therefore is an exposure apparatus that sustains the recent increase in the resolution.
A scanner transfers the pattern of a reticle onto a wafer by scanning them. Unless the focus between the reticle surface and the wafer surface during scanning is adjusted precisely, the imaging position deviates from a best focus position, blurring and deteriorating the formed optical image. That is, defocus occurs on the scanning image plane during scanning. Along with the recent increase in the NA of the exposure apparatus, the depth of focus (DOF) is decreasing. It is therefore becoming increasingly important to control the focus.
To determine a best focus position of the projection optical system, it is a common practice to print a certain pattern on a wafer or analyze an aerial image. Japanese Patent Laid-Open Nos. 2003-318090 and 2002-14005 respectively introduce these prior arts. In the former method, a test pattern is formed by exposure on a wafer coated with a resist, and the developed resist pattern is observed. In the latter method, an aerial image of a test pattern is formed near the object plane of the projection optical system, a change in the intensity of the aerial image is photo-electrically converted by scanning the aerial image by a knife edge or a slit opening, and the obtained signal is analyzed.
Unfortunately, it has been difficult for the prior arts to measure a deviation of the focus position during scanning in real time. The focus position measured by the method of printing a certain pattern on a wafer is the integrated value of deviations of the focus position which have occurred during scanning, and therefore is not a real-time focus position. Also, the optical image measured in the method of analyzing an aerial image is thought to be the one obtained by integrating deviations of the focus position because this method requires scanning to obtain optical image data, and therefore the measured focus position is not a real-time focus position, either. Both Japanese Patent Laid-Open Nos. 2003-318090 and 2002-14005 disclose no method of measuring a deviation of the imaging position during scanning. In other words, Japanese Patent Laid-Open Nos. 2003-318090 and 2002-14005 measure the light intensity distribution integrated after scanning exposure or that formed by exposure in a stationary state.